Ministry of Higher Education & Scientific Research University of Technology Chemical Engineering Department Enhancement of Reverse Osmosis Membranes Performance with Air Sparging Technique A Thesis Submitted to the Chemical Engineering Department Of The University of Technology In Partial Fulfillment of the Requirements for The Degree of Doctor of Philosophy in Chemical Engineering. By Talib Mohammad Naief (M. Sc. Chem. Eng.2001) May - 2009
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Ministry of Higher Education
& Scientific Research
University of Technology
Chemical Engineering Department
Enhancement of Reverse Osmosis Membranes
Performance with Air Sparging Technique
A Thesis
Submitted to the Chemical Engineering Department
Of The University of Technology
In Partial Fulfillment of the Requirements for
The Degree of Doctor of Philosophy in
Chemical Engineering.
By Talib Mohammad Naief
(M. Sc. Chem. Eng.2001)
May - 2009
الرحيم الرحمن هللا بسم
الماء نسوق أنا يروا أولم
فنخرج الجرز األرض إلى
مهم آع أن منه تأكل زرعا به
يبصرون أفال وأنفسهم
العظيم هللا صدق السجدة سورة
)27( االية
Certification
I certify that this thesis entitled (Enhancement of Reverse
Osmosis Membranes Performance with Air Sparging
Technique) was prepared under my linguistic supervision. It was
amended to meet the style of English Language.
Signature
Name: Asst. Prof. Dr. Ahmad AL-Beiruti.
Date: / / 2009
Certification of Supervisor We certify that the thesis entitled (Enhancement of Reverse
Osmosis Membranes Performance with Air Sparging
Technique) was prepared under our supervision as a partial
fulfillment of the requirements of the degree of Philosophy of
Doctorate in Chemical Engineering at the Chemical Engineering
Department, University of Technology.
Signature: Signature: Name: Prof. Dr. Mumtaz A. Yousif Name: Asst. prof. Dr. Qusay Fadhel Date: / / 2009 Date: / / 2009
In view of the available recommendations, I forward this
thesis for debate by the examination committee.
Signature
Asst.Prof. Dr. Kahlid A.Sukkar
Head of post graduate Committee
Department of Chemical Engineering.
Date: / / 2009
Dedication
Especially Dedicated To…. The memory of my Father To my mother with love My brothers and my sisters My Wife and Children Ibrahim and Masarra.
Talib
I
Acknowledgment
I would like to express my sincere thanks, gratitude and
appreciation to my supervisors Prof. Dr. Mumtaz A. Zabluk and
Asst. Prof. Dr. Qusay Fadhel Abdul Hameed for their kind
supervision, advice, reading and Criticizing the proofs of this
study.
First of all, I thanks god who offered me patience, power and
faith in a way that words cannot express.
My respectful regards to head of Chemical Engineering
Department at the University of Technology Prof. Dr. Mumtaz A.
Zabluk for his kind help in providing facilities.
My respectful regards to all staff of Physical Science Faculty
at the University of Complutense – Madrid - Spain. For their kind
help in providing facilities.
My grateful thanks to the staff of AL-Mansour Company for
their help in the experimental work.
My respectful regards to head of Material Engineering
Department at the University of Technology Prof. Dr. Ali H.Ataiwi
for his kind help in providing facilities.
My grateful thanks to Miss Nisreen, the chief of computer
laboratory.
My deepest gratitude and sincere appreciation goes to my
beloved family for their patience and encouragement that gave me
so much hopes and support that I feel short of thanks.
Talib
II
Abstract
In the present work, the central composite design (CCD) technique was
used to study the effects of various operating conditions such as: NaCl
concentration (15-45) gm/l, temperature (10-50) °C, flow rates (100-250) l/hr
and operating pressure (5-15) bar on the performance of the reverse osmosis
membrane (RO) type (Cellulose acetate, Sc-6200, spiral-wound model) were
studied by using the experimental design (Box Wilson) method. The objective
function (Response) was the flux of permeate and salt rejection. The coefficients
of the proposed model (second order polynomial model) were found, and then
the significant and non-significant parameters for the proposed model were
checked by the (F-test) method. In order to ensure a good model the (F-test) for
significance of the regression model was performed by applying the analysis of
variance (ANOVA). The maximum conditions for the proposed model by using
optimization program (Hook and Jeeves) were applied for the permeate flux and
salt rejection, where permeate flux was equal to (3.406 kg/m
2
An application of the gas-liquid two-phase flow for the permeate flux
enhancement during the (RO) membrane has been studied, through sparging air
in the system at different velocities by fixing the four variables at the maximum
conditions and then different velocities of liquid at fixed velocity of air. The
results of the experiments showed a positive effect of the constant gas-liquid
two-phase flow on the permeate flux and salt rejection, where the permeate flux
.hr) and salt
rejection was equal to (85%). In addition, it was found that the flux and the
rejection of NaCl solution throughout the reverse osmosis (RO) dependent on
feed concentration, feed temperature, feed flow rate and operating pressure in
the following sequence: feed concentration > operating pressure > feed
temperature > feed flow rate. A mathematical model was developed by (Jamal et
al, 2004) for prediction the permeate flux during reverse osmosis (RO)
membrane process has been applied and the results showed a good agreement
between the experimental data and the proposed model.
III
was increased from (3.406 kg/m2.hr) to (5.676 kg/m2
.hr) and salt rejection
increased from (85%) to (91%). It might be concluded from the analysis of the
experimental results based on the spiral wound filtration model that a two-phase
flow seemed to enhance the permeate flux and rejection with a slug flow pattern.
Thus allowing higher fluxes which led to an increase in permeate flux by a
factor (1.66).
IV
LLiisstt ooff CCoonntteennttss
Subject Page
Acknowledgments ........................................................... I
Abstract ............................................................................ II
List of Contents ......................................................... IV
Nomenclature ........................................................... VII
Chapter One – Introduction 1
1.1 Types of Membrane .............................................. 2
Desalination through the use of membranes was introduced in 1960s as an
alternative to distillation. A reverse osmosis membrane process is a physical
separation process, where salt is separated from seawater or brackish water to
produce drinking water.
Reverse osmosis (RO) is relatively new as compared to the distillation
processes. The first commercial unit was installed in Florida in 1971. The
reverse osmosis membrane separation process separates freshwater from
saltwater under high pressure where the freshwater passes through the
membrane layer while the salt content remains outside the membrane. The
amount of freshwater produced varies from 30 to 80% depending on the salt
content of the water, pressure and type of membranes used. Brackish water
membrane systems typically have higher recoveries and operate under lower
pressures, ranging from 225 psi to 375 psi. Seawater reverse osmosis systems
typically have lowers recoveries due to the higher salt content and their
operating range is typically 800 to 1200 psi (Beck, 2002).
Reverse osmosis is a process that transforms an unusable water supply
into a usable resource. It is capable of renovating a broad spectrum of feed
waters from municipal water supplies that need refinement for industrial
purposes to seawater that is refined into a potable water supply. Table 1.1 shows
the different types of feed water being processed by reverse osmosis units
(Mark, 1990). Seawater is considered to have nominal total dissolved solids
(TDS) content of 35,000 mg/l.
Chapter One Introduction
2
Table 1.1 Source of Reverse Osmosis Feed Water
Feed Water Capacity,MGD Percent of Total Sea Water 67.9 13.0
Waste Water 26.5 5.0 Brackish Water 429.6 82.0
Total 524.0 100.0 MGD = One million gallons per day
As of the end of 1984, the desalination of brackish water accounted for
82% of capacity. This is due to the fact that early reverse osmosis membranes
were incapable of single stage seawater desalination and, thus, they were limited
to brackish water desalination. Significant advances have been made in both the
flux and rejection capability of membranes and reverse osmosis is technically
able to desalt seawater in a single stage. In the recent past, it has been an
effective competitor to the distillation process in seawater desalination. In fact,
reverse osmosis is now beginning to replace existing distillation capacity in the
Middle East (Smith, 1985). Although reverse osmosis is a relatively new
technology, there is sufficient operating capacity in a number of varied
applications to warrant confidence in the process. From a technical and
economic point of view the process is capable of desalting a broad range of feed
waters from municipal water supplies to seawater. It has economic viability in a
large number of industrial applications.
1.1 Types of Membranes
Water treatment processes employ several types of membranes as shown
in Figure 1.1. They include microfiltration (MF), ultrafiltration (UF), reverse
osmosis (RO), and nanofiltration (NF) membranes. MF membranes have the
largest pore size and typically reject large particles and various microorganisms.
UF membranes have smaller pores than MF membranes and, therefore, in
addition to large particles and microorganisms, they can reject bacteria and
soluble macromolecules such as proteins. RO membranes are effectively non-
Chapter One Introduction
3
porous and, therefore, exclude particles and even many low molar mass species
such as salt ions, organics, and etc. NF membranes are relatively new and are
sometimes called “loose” RO membranes. They are porous membranes, but
since the pores are on the order of ten angstroms or less, they exhibit
performance between that of RO and UF membranes (Amjad, 1993), (Perry,
1997) and (Baker, 2004).
Figure 1.1 Range of nominal membrane pore sizes (Perry, 1997) 1.2 Membrane Modules
There are four main types of modules: plate-and-frame, tubular, spiral
wound, and hollow fiber shown in Figure 1.2, (Baker, 2004). The plate-and-
frame module is the simplest configuration, consisting of two end plates, the flat
sheet membrane, and spacers. In tubular modules, the membrane is often on the
inside of a tube, and the feed solution is pumped through the tube. The most
popular module in industry for nanofiltration or reverse osmosis membranes is
the spiral wound module. This module has a flat sheet membrane wrapped
around a perforated permeate collection tube (Baker, 2004). The feed flows on
one side of the membrane. Permeate is collected on the other side of the
membrane and spirals in towards the center collection tube. Hollow fiber
modules used for seawater desalination consist of bundles of hollow fibers in a
pressure vessel. They can have a shell-side feed configuration where the feed
passes along the outside of the fibers and exits the fiber ends. Hollow fiber
Chapter One Introduction
4
modules can also be used in a bore-side feed configuration where the feed is
circulated through the fibers. Hollow fibers employed for wastewater treatment
and in membrane bioreactors are not always used in pressure vessels. Bundles of
fibers can be suspended in the feed solution, and permeate is collected from one
end of the fibers (Baker, 2004).
Figure 1.2 Schematic of (a) plate and frame, (b) tubular, (c) spiral wound and (d) hollow fiber modules (Pelligrino and sikdar, 2004)
1.3 Application of Membrane Filtration
Applications of membrane filtration in water treatment can be divided into
two groups: (1) micro- and ultra filtration for the removal of particulate material
and micro organisms and (2) nanofiltration and reverse osmosis for the removal
of dissolved material and micro pollutants.
Although the type and the geometry of the membranes and modules are
different, the principle of membrane filtration is the same. The permeation rate
(flux) ranges from roughly 40 - 300 (l·mP
-2P·hP
-1P·barP
-1P) for microfiltration to 0.08 -
40 (l·mP
-2P·h P
-1P·barP
-1P) for reverse osmosis. At capacities up to several hundreds of
thousands cubic meters of drinking water per day, large membrane areas are
needed. Although careful selection of suitable membrane material (hydrophilic
or hydrophobic) is a necessity for successful application, other phenomena, like
mass transfer, back transport, diffusion and maldistribution are also important.
Chapter One Introduction
5
All these phenomena have a clear relation to the hydrodynamics in the
installation. In the design of membrane installations, these hydrodynamics play
an important role in the membrane (module design) and module arrangement
(plant design) to provide successful applications and limited energy
consumption and investment costs (Verberk, 2005).
1.4 Transport Phenomena in Membranes
The driving force for membrane filtration in water treatment is the
pressure gradient across the membrane. As a result of this driving force a
convective transport of material from the bulk to the membrane surface is
obtained. Solvent (water) permeates through the membrane and solutes
(dissolved and particulate material) are partly or completely retained by the
membrane. The retained dissolved solutes and particulate material accumulate in
a boundary layer at the membrane surface and a concentration build-up (in
time), the so-called concentration polarization, is observed see Figure 1.3. As a
result of the build-up of retained solutes at the membrane surface, the
permeation rate will decrease. The convective transport to the membrane surface
is balanced by the back transport from the membrane surface to the bulk. This
back transport is governed by diffusion or turbulence. When the convective
transport is equal to the back transport, a steady state situation is reached and the
permeate flux is constant in time. The back transport is influenced by the flow
conditions inside the membrane. Increase in back transport of rejected solutes
and particles by more turbulent flow conditions results in improvements in
permeation and selectivity. Concentration polarization can result in fouling.
Fouling is defined as: the process resulting in loss of performance of a
membrane due to deposition of suspended or dissolved substances on its
external surfaces, at its pore openings, or within it pores (Koros et al., 1996).
Fouling will always occur when particulate material is present in water.
Chapter One Introduction
6
Especially in micro- and ultra filtration the particulate fouling is a major point of
attention because rapid undesired flux decreases occur.
Figure 1.3 Concentration profiles of dissolved or particulate material and the main transport mechanisms in a membrane filtration process (Verberk, 2005)
1.5 Two-Phase Flow
Two-phase flow is the area of fluid mechanics that describes the flow of
mixtures consisting of two or more immiscible phases. Two-phase flow is the
simplest case of multi-phase flow. The different phases of multi-phase flow are
liquid, gas and solid. Two-phase flow is constantly met in our daily practice. For
example sandstorm, fog, snow and rain are natural examples of two-phase flow.
Two-phase flow is a well-known phenomenon in many industrial applications
(Wallis, 1969) and (Bachelor, 1989). Depending on the superficial velocities and
the pipe geometry different two-phase flow patterns occur, like bubble flow,
slug flow and annular flow. The segmented flow pattern slug flow is reported to
be very effective in small diameter tubes to increase heat and mass transfer rates
compared to single-phase flow. Slug flow was found to augment radial mass
transfer in reactors with catalytically active walls (Horvath, 1973). These results
suggest that slug flow could be a useful means to improve the efficiency of
Chapter One Introduction
7
many devices, which employ small diameter tubes and laminar flow by
enhancing radial mass transport or reducing axial dispersion. Such devices
include tubes with an absorbing wall for liquid chromatography or for selective
removal of solutes, reverse osmosis, or ultrafiltration systems having a semi-
permeable wall (Wallis, 1969). Water and air two-phase flow is already used in
water treatment processes. Well known examples are the water-air backwashing
of rapid sand filters and the water-air scouring of pipelines in the distribution
network.
From literature on heat and mass transfer, it is known that Taylor flow is a
specific two-phase flow pattern, results in an increased liquid-to-solid mass
transfer rate from bulk to wall compared to single phase liquid flow. This
increased mass transfer is caused by secondary rotating flows in the liquid slugs.
The increased mass transfer takes place at even lower pressure drops compared
to single phase flow (Kreutzer, 2003). In the automotive exhaust gas cleaning
Taylor flow is used to enhance mass transfer in monolith reactors. Monolith
reactors are ceramic structures of many parallel straight channels with a
diameter in the order of one millimeter. Based on structural configuration
membrane modules can be well compared with monoliths and the question
arises whether water-air two phase flow is also applicable in membrane filtration
processes to enhance the mass transfer. A major difference between monoliths
and membrane processes is the operational mode. In monoliths, the superficial
velocities are low compared to the velocities in membranes, so the extrapolation
of existing pressure loss equations, mass transfer relations and scale-up guide
lines are not directly possible.
Chapter One Introduction
8
1.6 Aim of the Present Work
This study focuses on investigating gas sparging as a technique to reduce
external fouling. In industrial membrane applications, membranes are typically
operated for several weeks before chemical cleaning. The main focus was on
monitoring the flux development with and without air sparging. Very rare work
was carried out in order to quantify the enhancement of permeates flux in
reverse osmosis membrane using sparging air. Slug flow is the most efficient
flow to enhance significantly the mass transfer in reverse osmosis membranes
when it is limited by particle deposit (Mercier et al; 1995). As a consequence,
this flow pattern has been chosen for the following study.
The aim of the present work can be summarized as follows:
1. Studying the effect of various operating conditions such as: concentration
(15-45) gm/l, temperature (10-50) °C, flow rates (100-250) l/hr and operating
pressure (5-15) bar ; on the performance of the reverse osmosis membrane ( type
(Sc-6200, spiral-wound model) by using NaCl as a feed solution. Flux of
permeate and salt rejection will be the main objective of this work.
2. Using experimental design (Box Wilson) methods in order to obtain the
proposed model (second order polynomial model) and its coefficient.
3. Obtaining maximum conditions for the proposed model by using optimization
program (Hook and Jeeves).
4. Sparging air in the system at different velocities of air after fixing the four
variables at the optimum conditions and different velocities of liquid at a fixed
velocity of air.
Chapter Two Theoretical Concepts and Literature Survey
9
Chapter Two
Theoretical Concepts and Literature Survey
Osmosis is a natural phenomenon in which a solvent (usually water)
passes through a semi permeable barrier from the side with lower solute
concentration to the higher solute concentration side. As shown in Figure 2.1a,
water flow continues until chemical potential equilibrium of the solvent is
established. At equilibrium, the pressure difference between the two sides of the
membrane is equal to the osmotic pressure of the solution. To reverse the flow
of water (solvent), a pressure difference greater than the osmotic pressure
difference is applied see Figure 2.1b; as a result, separation of water from the
solution occurs as pure water flows from the high concentration side to the low
concentration side. This phenomenon is termed reverse osmosis (it has also been
referred to as hyper filtration). A reverse osmosis membrane acts as the semi
permeable barrier to flow in the RO process, allowing selective passage of a
particular species (solvent, usually water) while partially or completely retaining
other species (solutes). Chemical potential gradients across the membrane
provide the driving forces for solute and solvent transport across the membrane:
- Δ μ Rs R , the solute chemical potential gradient, is usually expressed in terms of
concentration; and - Δ μ RwR , the water (solvent) chemical potential gradient, is
usually expressed in terms of pressure difference across the membrane
(Bhattacharyya and Williams, 1992b).
Chapter Two Theoretical Concepts and Literature Survey
10
Figure 2. 1 Schematic of Osmosis (a) and Reverse Osmosis (b) Phenomena (Bhattacharyya and Williams, 1992b)
Chapter Two Theoretical Concepts and Literature Survey
11
2.1 Reverse Osmosis Process Description and Terminology
The reverse osmosis process is relatively simple in design. It consists of a
feed water source, feed pretreatment, high pressure pump, reverse osmosis
membrane modules, and, in some cases, post treatment steps. A schematic of the
reverse osmosis process is shown in Figure 2.2a.
The three streams (and associated variables) of the reverse osmosis
membrane process are shown in Figure 2.2b the feed; the product stream called
permeate; and the concentrated feed stream, called the concentrate or retentate.
The water flow through the membrane is reported in terms of water flux, JRwR,
where:-
Solute passage is defined in terms of solute flux, Js:
(2.2)
Solute separation is measured in terms of rejection, defined as:
(2.3)
The quantity of feed water that passes through the membrane (the permeate) is
measured in terms of water recovery, r, defined for a batch RO system as
R R(2.4)
Where is permeate volume (mP
3P) and is feed volume (mP
3P).
And for a continuous system as
(2.5)
Where is permeate flow rate and is feed flow rate
In a batch membrane system, water is recovered from the system as the
concentrate is recycled to the feed tank; as a result, if the solute is rejected the
Chapter Two Theoretical Concepts and Literature Survey
12
feed concentration (cRfR) continuously increases over time. For a continuous
membrane system, fresh feed is continuously supplied to the membrane. Water
flux is sometimes normalized relative to the initial or pure water flux (JRwoR) as
JRwR/JRwoR or as flux drop, defined by:
(2.6)
The pressure difference between the high and low pressure sides of the
membrane is denoted as ΔP while the osmotic pressure difference across the
membrane is defined as Δπ; the net driving force for water transport across the
membrane is (ΔP - σΔπ), where σ is the Staverman reflection coefficient.
(Gekas, 1988) reviewed the standardized terminology recommended for use to
be used for describing pressure-driven membrane processes, including reverse
osmosis.
Figure 2.2 Schematic of (a) RO Membrane Process and (b) RO Process Streams (Gekas, 1988)
Chapter Two Theoretical Concepts and Literature Survey
13
2.2 Theory
The theory governing fluid transport through membranes is often
expressed as follows (Bird et al; 2002):
Where NR
A R
is the mass flux of component A through the membrane (mass per
time per area), ρR
A R
is the mass density of component A, v is the mass average
velocity of the fluid through the membrane, DR
AB R
is the effective diffusion
coefficient of component A in the membrane, and ∇ ρRAR is the mass density
gradient. In membranes where pore flow contributes significantly to flux,
Darcy’s Law is often used to characterize the mass average velocity (Bird et al;
2002):
Where κ is the Darcy Law permeability of the medium, μ is the fluid viscosity,
∇p is the pressure gradient (i.e., the rate of pressure change with respect to
position), ρ is the solution density and g is the gravity vector. Introducing
equation (2.8) into equation (2.7) and restricting transport to only the x-
direction, which will typically be the direction perpendicular to the membrane
surface, and by neglecting gravity, yields:
The first term in equation (2.9) represents mass flux due to pressure-driven
convection through pores, and the second term represents flux due to diffusion.
Diffusion through porous membranes is typically negligible relative to
convection. In this case, the flux is directly proportional to the pressure gradient
(2.7)
(2.8)
(2.9)
Chapter Two Theoretical Concepts and Literature Survey
14
across the membrane. The applied pressure difference across the membrane
which often called the transmembrane pressure difference is the driving force
governing transport of liquid through a porous membrane.
In applying the convective term of equation (2.9) to transport through UF
and MF membranes, the permeability, κ, depends often in a complex way, on
factors such as the porosity and the tortuosity of the membrane. Tortuosity, τ, is
the ratio of the average length of the “tortuous” path that the fluid must travel to
pass through the membrane to the membrane thickness. For example, a
cylindrical pore perpendicular to the surface has a tortuousity of one. Most
phase inversion membranes have tortuousities from 1.5 to 2.5 (Baker, 2004).
Porosity, ε, is the void fraction of the membrane. UF and MF membrane
porosity typically ranges from 0.3 to 0.7 (Baker, 2004).
Since RO membranes are effectively non-porous, the transport of a
molecule across the membrane is diffusion controlled. This means that the
second term of equation (2.9) controls the flux across the membrane. Water
molecules desorb into the upstream face of the membrane, diffuse down the
chemical potential gradient across the membrane, and then desorbed from the
downstream face of the membrane. The second step, diffusion through the
membrane, is the rate-determining step in water transport across the membrane.
This mechanism of mass transport across membranes is commonly referred to as
the “solution- diffusion” modelP
P(Bird et al; 2002).
Beginning with the more general model of mass transport being driven by
chemical potential gradients rather than concentration gradients, the solution-
diffusion transport equation for reverse osmosis can be derived P
P(Bird et al;
2002), (Baker and Wijmans,1995) :
Where NR
Aw R
is the water flux through the membrane, Δp is the transmembrane
pressure difference, Δπ is the difference in osmotic pressure between the feed
(2.10)
Chapter Two Theoretical Concepts and Literature Survey
15
and the permeate, and L is a constant describing the physical characteristics of
the membrane itself. Within the context of the solution-diffusion model used to
describe transport in nonporous films, L is given byP
P(Baker and Wijmans, 1995):
Where D is the water diffusivity in the membrane, S is the water solubility in the
membrane, V is the molar volume of water, R is the ideal gas constant, T is the
ambient temperature, and l is the membrane thickness. A complete derivation
can be found in the Baker and Wijmans review of the solution-diffusion model
(Baker and Wijmans, 1995) and in Paul’s recent re-examination of the solution-
diffusion model for reverse osmosisP
P(Paul, 2004).
As can be seen from equation (2.10), osmotic pressure of the feed and
permeate solutions plays a role in the separation. Osmotic pressure is the
pressure needed to cause a solvent (water) to leave a solution (seawater, waste
water, etc.) and permeate through the membrane. For an ideal solution, with
complete dissociation of salt ions, osmotic pressure is defined as P
P(Freeman,
1995):
Where π is the osmotic pressure, C is the salt ion concentration, R is the ideal
gas constant, and T is the solution temperature. The salt ion concentration, C, is
given by the number of ions in solution per gram of water divided by the
specific volume of water. Table 2.1 presents the osmotic pressure for several
solutions pertinent to water treatment applications. Table 2.1 Typical osmotic pressure values for solutions at 25°CP
P(Freeman, 1995).
Solute Concentration
(mg/l) Osmotic Pressure
(psi) NaCl NaCl
Brackish water Sea Water
2,000 35,000
2,000-5,000 32,000
23 397
15-39 339
Chapter Two Theoretical Concepts and Literature Survey
16
In reverse osmosis, salt transport across a membrane is as important as
water transport. However, unlike water flux, which is driven by both applied
transmembrane pressure and osmotic pressure, the salt flux is only a function of
salt concentrationP
P(Baker and Wijmans, 1995):
Where NR
s R
is the salt flux through the membrane, B is the salt permeability
constant describing the physical characteristics of the membrane, R R
is the salt
concentration in the feed solution, and R R
is the salt concentration in the
permeate solution. Analogous to L in the solution-diffusion equation, B is given
byP
P(Baker and Wijmans, 1995):
Where DR
s R
is the salt diffusivity in the membrane, KR
s R
is the salt partition
coefficient, and l is the membrane thickness. However, instead of reporting salt
flux values, most membrane performance specifications provide salt rejection
values.
Furthermore, water flux and salt flux depend on each other. Equation (2.15)
relates the water flux, NR
AwR
, to the salt flux, NR
sRP
P(Riley et al; 1967):
Where CR
w R
is the water concentration in permeate and R R
is the salt concentration
in permeate. By substituting equation (2.10) and equation (2.13) into equation
(2.15) and rearranging terms, the following expression for rejection may be
derivedP
P(Riley et al; 1967):
Chapter Two Theoretical Concepts and Literature Survey
17
Equation (2.16) relates salt rejection to the physical properties of the
membrane (which influence L and B), the applied transmembrane pressure
difference, and the osmotic pressure difference between permeate and the feed.
Equation (2.16) allows one to predict the salt rejection of the membrane based
on the experimental conditions and the membrane properties.
2.3 Factors Affecting Flux
2.3.1 Operating Parameter
There are four major operating parameters that affect the flux: (1)
pressure, (2) feed concentration, (3) temperature, and (4) turbulence in the feed
channel (flow rate).
1. Pressure
The major parameter that directly influences the energy consumption of
the RO plant is the feed pressure. The higher feed pressure is the higher energy
consumption of the plant. The permeate production strongly depends on the feed
pressure. Whereas the feed pressure influences the two primary operating
parameters, productivity and product water conductivity. The pressure drop
affects the mechanical stability of the RO equipment. In a spiral-wound system,
the pressure drop translates into a force directly on the membrane element and
also on the product tube (Herold ., 2001).
2. Feed Concentration
The film theory model states that the flux will decrease exponentially with
increasing feed concentration. This relationship should hold true regardless of
the type of flow or degree of turbulence or the temperature (Munir, 1998).
3. Temperature
In General, higher temperatures will lead to higher flux in both the
pressure controlled region and in the mass transfer-controlled region, this
assumes there are no other unusual effects occurring simultaneously, such as
fouling of the membrane due to precipitation of insoluble salts at higher
Chapter Two Theoretical Concepts and Literature Survey
18
temperatures or denaturation of proteins or gelatinization of starch at higher
temperatures. In the pressure controlled region, the effect of temperature on flux
is due to its effect on fluid density and viscosity. Activation energies for both
flux and viscosity are similar in the region of 20-50°C, about 3400 kcal/mole. In
practical terms,it will take a temperature rise of 30-45°C to double the flux
(Munir, 1998).
4. Flow Rate and Turbulence
Turbulence, whether produced by stirring, pumping the fluid, or vibrating
the membrane, has a large effect on flux in the mass transfer-controlled region.
Agitation and mixing of the fluid near the membrane surface "sweep" away the
accumulated solute, reducing the hydraulic resistance of the "cake" and reducing
thickness of the boundary layer. There is also a belief that extremely high shear,
such as that obtained with thin - channel and rotary device, actually reduce the
thickness of the "gel" layer. In any case, this is one of the simplest and most
effective methods of controlling the effects of concentration polarization (Munir,
1998).
2.3.2 PH of Feed
The pH of the feed water must be measured and controlled in reverse
osmosis desalination of water for several reasons. The first is to prevent CaCOR3R
precipitation. The second reason is to maximize the life of membrane of the
cellulose acetate type. Cellulose acetate is an ester which reacts slowly with
water to form an alcohol and an acid. The rate of this reaction, which is called
hydrolysis, is dependent on both pH and temperature.
The minimum hydrolysis rate at a particular temperature occurs at a pH of
(4.5-5) as the hydrolysis continues, the passage through the membrane of both
water and salt increases. The salt passage increases the product water
conductivity. In the operation of cellulose acetate membrane, the pH is reduced
to pH 6 or less in order to slow the hydrolysis rate to a value which permits long
term operation (Mindler & Epstein, 1986).
Chapter Two Theoretical Concepts and Literature Survey
19
2.4 Flux Decline in Membranes and Strategies to Reduce Fouling
The main problem in membranes, where very high permeation fluxes and
complicated feeds containing a broad particle size distribution are present, is
concentration polarization and subsequent fouling.
Concentration polarization is the build-up of rejected solutes at the liquid
boundary layer near the membrane. If there is a certain degree of mixing,
diffusion and inertial lift of the rejected components can result in a backtransport
to the bulk. Convection of particles towards the module exit due to the crossflow
will then limit their accumulation on the membrane. If the transport of the
rejected components back to the bulk solution is not fast enough, deposition of
material on or in the membrane occurs. This process is known as fouling as
shown in Figure 2.4.
Figure 2.4 Concentration polarizations in a reverse osmosis membrane system. (a) Before membrane is fouled and (b) after membrane is fouled.
Particle deposition is a process that is governed mainly by the
hydrodynamic forces acting on the particle near the membrane surface (Zeman
and Zydney, 1996). The important forces on the particle include the viscous drag
force performed by the flowing fluid, the hydrodynamic lift force arising from
the inertial interactions between particle and solid boundary and diffusion forces
due to Brownian motion. The dominant force for small particles is Brownian
Chapter Two Theoretical Concepts and Literature Survey
20
motion, which is responsible for the equilibrium state in macrosolute-membrane
interactions. As the particle size increases, the importance of Brownian diffusion
decreases since it becomes too slow. Deposition of bigger particles will occur
when the forces towards the membrane surface are greater than the repulsive
interactions between particles, inertial lift forces and shear-induced diffusion.
The analysis of the flux decline due to particle deposition is of special
importance since it can provide some insight to the phenomena that take place
during microfiltration. Depending on the solute and the process conditions,
different blocking mechanisms that explain the flux decline during membrane
filtration has been developed (Hermia, 1982), (Bowen et al, 1995) and
(Wessling, 2001):
– Complete blocking (pore blocking)
– Standard blocking (pore narrowing)
– Intermediate blocking (long term deposition)
– Cake formation (gel/cake layer)
These mechanisms are schematically shown in Figure 2.5. Pore blocking (a) is
caused by rejected particles bigger than the membrane pores. This mechanism
assumes that each particle arriving at the membrane contributes in the complete
inactivation of one or more pores, causing a dramatic flux decline. Pore
narrowing (c) is mostly caused by smaller components that can adhere to the
internal pore wall, accumulate or bridge and finally clog the pore. Intermediate
blocking (b) is the stage preceding cake layer formation (d). A cake layer is
formed when each particle arriving to the surface accumulates on each other,
thus completely blocking the membrane surface. The flux decline due to
particles can be governed by one mechanism but it can also be a combination of
more than one. Although these mechanisms are developed for the filtration of
proteins, they are also valid for different types of solutes.
Chapter Two Theoretical Concepts and Literature Survey
Equation (3.30) and then (3.34) can be solved with the help of fourth order Runge-
Kutta technique.
For the determination of model constant the six model constants and two
initial conditions were used in the simulation program. The initial conditions are
feed concentration Cfo and feed volume Vfo. Membrane surface area Sa and
operating pressure gradient ΔP are two model constants that represent design
variables, the solvent (water) concentration is Cwp
Experimental data for aqueous salt (NaCl) solution taken at different concentrations of the feed water is used to verify the model.
.
The constants and initial conditions for model simulation are shown in Table (3.1).
Chapter Three Mathematical Model
45
Table (3.1) Parameter Values for Model Simulation
Parameter Value
Initial solute feed concentration,
Cfo, kg/m
15 kg/m3
3
Initial feed Volume, Vfo ,m 1.5 m3 3
Solvent permeate concentration,
Cwp , kg/m
1000 kg/m3
3
Membrane surface area, Sa, m
(Sc-2600, spiral wound model
2 35.2 m2
Membrane pressure gradient, ∆p, kg/m.h
3.22×10 P
13
Solvent permeability constant ARwR, h/m
4.88×10 P
-13
Solute permeability constant,
BRsR, m/h
1.13×10 P
-4P
Osmotic pressure to solute
concentration ratio, ψ , m P
2P/h P
2
1.3608×10 P
12
Chapter Four Experimental Work
46
Chapter Four
Experimental work
The present study includes the achievement of experimental work through
central composite rotatable design method to create samples of different artificial
flux and rejection. The experimental work of desalination sample water was carried
out in two stages. The first stage treatment of salt water by laboratory scale of
reverse osmosis membrane was carried out in Spain - Madrid - University of
Complutense – Faculty of Physics, Department of Applied Physics. In this stage
the removing of TDS was within the allowable requirement range and the result
was analyzed theoretically.
The operating conditions using reverse osmosis membrane process were
commenced with the following ranges:
A. Concentration of Feed (15-45) g/l.
B. Temperature of Feed (10-50) °C.
C. Flow rate of Feed (100-250) l/hr.
D. Operating Pressure (5-15) bar.
The second stage includes injection air with salted water for enhancement
permeates flux and rejection after obtaining the maximum conditions from the first
stage. This stage was carried out by laboratory scale of reverse osmosis membrane
system which located at in AL-Mansuor Company - Ministry of Industry and
Minerals - Baghdad - Iraq.
This chapter explains and views in details the experimental part of this work.
It includes the description of the experimental rig in order to study the behavior of
the process and measuring the experimental data.
Chapter Four Experimental Work
47
4.1 The Experimental System
Figure (4.1) and Figure (4.2) show the schematic diagram and photograph of
the Reverse Osmosis membrane system used in this study. The Reverse Osmosis
membrane system consists of the following items:-
1. Electrical Pump
It’s a horizontal rotodynamic high pressure pump type (GE motors &
industrial system). The electrical pump is provided to aid up the makeup NaCl
solution into the reverse osmosis membrane module and maintain it at the required
level from the pressure and flow rate. The specifications of this type were (HP ½ ,
Hz 60/50 , V 100-120/200-240 , PH 1, RPM 11725/1425 , A 7.1-7.2/3.4-3.6 , gage
of pressure read 300 Psi as a maximum) assembled in Mexico.
2. Membrane Module
Type of membrane was osmonics (Cellous acetate, Sc-6200, spiral-wound
model, DESAL™ Membrane Products, made in USA) the surface area of membrane
1.12 m2
3. Feed Tank
. This reverse osmosis membrane module can be used to extract fresh water
from salt water but it requires a lot of pressure.
The feed solution was prepared by dissolving NaCl salt in distilled water
according the concentration of each experiment and poured in the feed tank which
was cylindrical glass vessel with total capacity 5 liter (made in Germany).
4. Thermostat
To maintain the temperature for each experiment constant, a rectangular
container of bathwater was filled with water and by circulating this water inside the
jacket of the feed tank while the temperature remains constant during the time of
the experiment.
Chapter Four Experimental Work
48
Figure (4.1) the schematic diagram of the experimental rig
Rotameters
Concentrated Manometers
Permeate
Conductivity Monitor
Pressure Controller
Thermometer
Filter
Feed Tank
Thermostat
High pressure pump
Low pressure Pump
Membrane
Chapter Four Experimental Work
49
Figure (4.2) the general view of the experimental rig
4.1.1 Measuring Devices
Different measuring devices were used through the experimental
investigation of this study, these devices are as follows:-
1. Pressure Gauge
This device was used to measure the transmembrane pressure across the
reverse osmosis membrane module and by circulating two circular valves to the
right or left direction in order to obtain the exact pressure and flow rate at the same
time.
2. Temperature Measurement
In order to measure the temperature of the feed solution during the running
time of the experiment, sensible device type (Temp.-MeBgreät Pt100, PHYWE.
11759.93, and Nr 000719) to be inserted inside the feed tank, was used.
Chapter Four Experimental Work
50
3. Conductivity Meter
Metrohm Ω 712 Digital conduct meter types 1.712.0010 and Nr.10191 was
used to measure the conductivity of feed solution and permeate water for each
sample (made in Switzerland).
4. Rota meter
Is an instrument used for measuring the flow rate of the feed solution.
5. Electronic Balance
Eventually, digital electronic balance from AD instrument LTD. Type GF-
1200-EC, max.1210g, e = 0.01, min.0.02g, d = 0.001g and has Ac adapter DC 12v
with digital means was employed to measure weight of permeate water during flux
calculation (made in Japan).
4.1.2 Experimental Procedures
1. Preparation of salt solution as a test media was achieved to the desired
concentration according to the number of experiments in a table of experimental
design and the checking of the desired concentration was performed by
conductivity meter. Hereinafter, the prepared solution is poured in the glass feed
tank.
2. The electrical current was switched on to operate the experimental rig and the
calibration for the pump was done to obtain the desired pressure and flow rate
while the temperature of salted solution was controlled by thermostat.
3. After adjusting the operating conditions, predetermined a condition that has
been already designed was applied according to the central composite rotatable
design of Box-Wilson. Accordingly, the flow rate measurement, water bath
temperature, concentration of the feed solution and eventually the pressure were
justified.
Chapter Four Experimental Work
51
4. Each experiment that was already carried out according to the previous pre
designed conditions, by measuring the volumetric flow rate of permeate water was
followed in order to calculate the flux and rejection. After measuring the
volumetric flow rate of permeates water, the samples were weighted by electronic
digital balance type GF-1200-EC.
4.2 Experimental Design
The experimental work of the present study involves the investigation of the
four variables such as; concentration, temperature, flow rate and pressure of feed
solution. Experimental planning was applied as recommended by Cochran
(Cochran, 1957) to reduce the number of experiments that would give sufficient
information in order to conclude the extent of the effect of each variable on the
membrane efficiency. The application of the experimental design for planning the
required experiment to examine the system, will extract the information from pre-
existing data by using a statistical method in order to interpret the results in a
regular form with the minimum number of observation. (Cochran and Cox, 1957).
The experimental design technique consists of two parts:-
1. Planning the experiments according to a specified plan, taking into account
the description of the variables value in the plan by a coded form.
2. Achieving the regression analysis for the specified set of runs in the plan,
also taking into account the coded form of the objective function regarding
each experiment in the set.
4.2.1 Fitting the Second Order Model
An experimental design for fitting the second-order model must have at least
five levels for each factor so that the model parameters can be estimated (i.e.
variables are usually called factor and the particular value of the variable is called
Chapter Four Experimental Work
52
the level). There are many techniques for the application of experimental planning,
such as factorial design, fractional design and box-Wilson. The proper technique
for planning a system of more than three variables "central composite rotatable
design" the total number of treatment combination is equal to (2K +2K +1), where
(K) is the number of variables, plus one additional further treatment that takes the
lack of fit and experimental error into account.
4.2.2 Central Composite Rotatable Design
This design consist of a 2K fractional (i.e. coded to the usual ± 1 notation) augmented by 2K axial points [i.e. (± ,0,0,…..,0),(0, ±, 0,…..,0), (0,0, ±,…..,0), …., (0,0,…., ±,) and center points (0,0,0,…..,0)].
A preliminary step is to set up the relationships between the coded levels and
the corresponding real variables, these relationships are as follows (Box and
George., 1978):
( )1.4][.min
−
=−
KXXXXX
center
centeractualCoded
The operating conditions of Reverse Osmosis membrane system are as follows:-
1. Concentration of Feed (15-45) g/l.
2. Temperature of Feed (10-50) °C.
3. Flow rate of Feed (100-250) l/hr.
4. Operating Pressure (5-15) bar.
The central composite rotatable design of four variables is used. The coded
levels are related to the real process values of these variables as follows:
Chapter Four Experimental Work
53
(4.5)2.5
10PX
(4.4)37.5
175FX
(4.3)10
30TX
(4.2)7.5
30CX
4
3
2
1
−=
−=
−=
−=
Where: [C] is the concentration of feed in (g/l), [T] is the operating temperature in
(°C), [F] is the flow rate of feed in (l/hr) and [p] is the operating pressure in (bar).
The working range of the coded and corresponding real variables is listed in
Table 3.1. Thirty one experiments were carried out in a sequence shown in Table
3.2 where the coded values +2, -2, 0 present the maximum, minimum and average
values respectively.
Table (4.1): Working range of coded and corresponding real variables Coded level
Concentration (g/l) Temperature (C⁰) Flow rate(L/hr) Pressure(bar)
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